Protein intake affects levels of G-protein subunits G alpha i2, G alpha i3, and G beta in rat glomerular membranes.

Using toxin-catalyzed ADP-ribosylation and specific immunoblots we examined whether the mass of G-protein subunits, G alpha s, G alpha i (includes G alpha i2, G alpha i3, and G alpha 0) and G alpha beta, in glomerular membranes was altered by dietary protein intake. ADP-ribosylation catalyzed by cholera toxin (CT) or pertussis toxin (PT) detected significant amounts of G alpha s or G alpha i in glomerular membranes from rats fed a low (6% casein) or a high (40% casein) protein diet. There was no significant difference in G alpha s content between glomerular membranes from low or high protein-fed rats. However, the amounts of G alpha i were significantly lower in glomerular membranes from rats fed a high protein diet when compared to glomerular membranes from rats fed a low protein diet. Two isoforms of immunoreactive G alpha s, 45 and 52 kDa proteins, were detected in glomerular membranes. The predominant isoform of G alpha s was a 52 kDa protein. As with ADP-ribosylation, immunoblots showed no significant difference in G alpha s content between glomerular membranes obtained from the two diet groups of rats. Also, immunoreactive G alpha i2, G alpha i3 and G beta were present in glomerular membranes. The mass of G alpha i2 and G alpha i3 was significantly lower in glomerular membranes of rats fed a high protein diet than in those of rats fed a low-protein diet. The decreased mass of total G alpha i, that is G alpha i2 and G alpha i3, was comparable to that seen with PT-catalyzed ADP-ribosylation.(ABSTRACT TRUNCATED AT 250 WORDS)